Certainly by mid 70s both The Supremes and Diana Ross were no longer having albums packed with covers of hits. When they did record a cover, it was more likely the song was an interesting album song on another artist's album that would work well for them [[for example Don't Let My teardrops bother you or Too Shy to Say).
Just thinking off the top of my head in regards to my own music collection, I would say 1980 was a turning point, where a lot of albums were focusing on exclusively or overwhelmingly original content. A lot of albums in my collection from the 1970s often have a good number of covers on them. But often the approach was different.
In the 60s I think a bunch of covers on a "pop" album were for two reasons:
1)Covering the American songbook was popular among genres and so those in the pop field figured to do the same but with a twist, the twist being covering songs that were contemporarily popular.
2)A lot of albums were packaged quickly to capitalize on the popularity of a singer's hit, and thus on the singer themselves, and it was just easier to re-record popular songs of the day, often note for note, rather than track down original music, and bam! the album's done.
What I love about a lot of the covers on albums from the 70s, especially from soul acts, is that they'd take a popular song and put their own spin on it. They didn't necessarily have to turn the song on it's head [[for example, "Aint No Mountain High Enough") but they attempted to make the song their own [[for example New Birth's "Fire and Rain"). A lot of the Supremes' Motown covers in the 60s hardly ever had the charisma and charm of the originals. The tracks were often unimaginative and then it's like the girls were thrown into the studio and told to sing without any rehearsal or need to be creative. There are obvious exceptions to this of course, but this is largely how I feel about Supremes' cover songs, particularly in the 60s.
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